The following article was written by Joe Munch, and published in MD Anderson's OncoLog.
OncoLog, January 2013, Vol. 58, No. 1
Myriad advances have been made in the treatment of breast cancer, and cures are achieved in many patients. However, there are still patients whose cancer recurs, and most of these patients will die of their disease. This indicates a need for other therapies that can be used to prevent recurrent disease. One potential option is breast cancer vaccines.
“Breast tumors are made up of so many different types of cells that we have to use many different drugs and therapies to treat them,” said Jennifer Litton, M.D., an assistant professor in the Department of Breast Medical Oncology at The University of Texas MD Anderson Cancer Center. “Vaccines come at the cancer in a totally different way than our current systemic therapies do.” Vaccines thus may augment the effects of adjuvant treatments currently used to forestall recurrence.
Potential clinical role
Several types of adjuvant therapy are used to prevent breast cancer from returning; the therapy or combination of therapies used depends on the individual patients and their disease. For example, radiation therapy is used for patients who have undergone breast-conserving surgery, and chemotherapy may benefit patients at high risk of recurrence. Hormonal therapy with tamoxifen or an aromatase inhibitor is used in patients with estrogen receptor–positive disease, and immunotherapy with trastuzumab is used in those with tumors that highly express human epidermal growth factor receptor 2 (HER2).
Today, several clinical trials are evaluating the use of breast cancer vaccines—not as an alternative to currently available preventive therapies for recurrent disease but as an additional adjuvant therapy.
“This is a novel approach specifically for people who want another form of therapy to decrease the chance of the cancer coming back,” Dr. Litton said. “People are looking for something extra that may improve their outcome but doesn’t expose them to a lot of extra toxicity.”
Peptide vaccines
Cancer vaccines stimulate patients’ immune systems to recognize and kill tumor cells. The vaccines consist of a tumor-associated antigen that, once introduced into a patient’s body, elicits an immune response. Several systems have been devised to deliver tumor-associated antigens into the body, including whole-cell vaccines, viral vector vaccines, and dendritic cell vaccines, which are custom made from the patient’s own white blood cells. The only therapeutic cancer vaccine currently approved by the U.S. Food and Drug Administration is sipuleucel-T (Provenge), a dendritic cell vaccine used in men with metastatic hormone-refractory prostate cancer.
The breast cancer vaccines being investigated at MD Anderson are of a fourth type, peptide vaccines. Peptide vaccines are made by taking a small amino acid sequence (peptide) from a tumor-associated antigen. The tumor-associated antigen most frequently used in breast cancer vaccines is the HER2 oncoprotein, which promotes tumor growth.
Once taken from the antigen, the peptide is mixed with an immunoadjuvant to help stimulate an immune response. The immunoadjuvant used in the trials being conducted at MD Anderson is granulocyte-macrophage colony-stimulating factor (GM-CSF), which has been used primarily to treat neutropenia in transplant recipients.
When the peptide–GM-CSF combination is injected, GM-CSF stimulates the dendritic cells in the area of injection to take up and process the peptide so that it can be better presented to the immune system. The length of the peptide dictates the type of immune cell it stimulates.
Current clinical studies
Several HER2-derived peptide vaccines are being studied in clinical trials at MD Anderson. Although the vaccines are based on a HER2 peptide, they have the most benefit in the 60% of breast cancer patients with low HER2 expression (1+ or 2+ by immunohistochemistry).
Phase III trial of E75
The E75 vaccine (NeuVax) is the most studied of the HER2-derived peptide vaccines. The 9-amino-acid peptide E75 binds with major histocompatibility complex (MHC) class I molecules to stimulate CD8-positive T cells; when these T cells recognize a target as foreign, they attack it and release cytotoxic enzymes to kill it. Because E75 is an MHC class I peptide, the vaccine works only in patients whose cells are positive for human leukocyte antigen (HLA)-A2 or HLA-A3; only cells with those HLA types will present the peptide on the cell surface to activate T cells.
In May 2012, Elizabeth Mittendorf, M.D., Ph.D., an assistant professor in the Department of Surgical Oncology, and her colleagues published the 24-month landmark analysis of their phase I and II trials of E75. The group’s findings opened the door to the phase III PRESENT (Prevention of Recurrence in Early-Stage, Node-Positive Breast Cancer with Low to Intermediate HER2 Expression with NeuVax Treatment) study, currently the only phase III trial of a breast cancer vaccine. Dr. Mittendorf is the overall principal investigator of the multinational study.
This randomized, double-blind, placebo-controlled trial will enroll approximately 700 breast cancer patients who were rendered disease free following standard treatment. Patients must be positive for HLA-A2 or HLA-A3 and have had cancers that were scored as HER2 1+ or 2+ by immunohistochemistry. The vaccine will be given once a month for 6 months and then given as a booster inoculation every 6 months thereafter through 3 years. Because GM-CSF causes inflammation at the injection site, it will be given to patients in each study group, serving as the immunoadjuvant for the vaccine group and as an active placebo for the control group. The primary endpoint of the study is 3-year disease-free survival.
Positive results from this trial, researchers hope, would eventually lead to indications for the E75 vaccine in the routine care of breast cancer patients. “We are all cautiously optimistic—and excited—as we wait for the results. If they do show that E75 has significant benefit, it could be an amazing opportunity for our cancer patients,” Dr. Litton said.
Phase II trial of GP2 and AE37
The GP2 vaccine works in much the same way as the E75 vaccine. Like E75, the GP2 peptide is 9 amino acids long and binds to MHC class I molecules to stimulate CD8-positive T cells; thus, the vaccine works only in patients who are positive for HLA-A2 or HLA-A3. In contrast, the AE37 peptide, which is longer than the E75 and GP2 peptides, binds to MHC class II molecules and stimulates CD4-positive T cells, thereby eliciting a more robust immune response. Although MHC class II peptides can be HLA-restricted, AE37 is a promiscuous peptide, meaning that blood cells of almost any HLA type can present it. In addition, the AE37 peptide is paired with the Ii-Key protein, which enhances the presentation of the peptide to the immune system.
Both the GP2 and AE37 vaccines are being investigated in an ongoing phase II trial to determine whether the individual vaccines can prevent the recurrence of node-positive or high-risk node-negative breast cancer. Patients are sorted into groups depending on their HLA status and then randomly assigned to receive the appropriate vaccine plus GM-CSF or GM-CSF alone (as the control).
The AE37 trial’s planned interim analysis revealed that at a median of 22 months, the recurrence rate in the vaccinated patients was 10.3%, whereas the recurrence rate in the control group receiving only GM-CSF was 18.0%. The difference represents a 43% reduction in recurrence rate.
“These data are encouraging,” Dr. Mittendorf said. “Obviously, we need longer follow-up, and we need to finish accrual in the trial, but the data suggest that it is reasonable to look forward to investigating the AE37 vaccine in a phase III setting.”
The interim results for the GP2 vaccine are not yet available.
Potential benefits
One of the benefits of peptide vaccines such as those being investigated at MD Anderson is that they can be given “off the shelf.” This makes them more convenient and less expensive than the custom-made dendritic cell vaccines.
Dr. Litton, who has referred a number of patients to the breast cancer vaccine trials, said that patients’ enthusiasm about participating in a vaccine trial has been overwhelmingly positive. “Some patients tell me that they feel empowered by using their own bodies, their own immune systems, to fight the cancer,” she said.
But the main reason the trials are so popular with patients is that the vaccines offer a potential anticancer benefit with very little risk of toxicity. Most patients have a grade 1 or 2 local toxic response, which means redness at the injection site; and some patients experience grade 1 or 2 systemic symptoms, mostly in the form of minor flu-like symptoms for 4–6 hours after receiving the vaccine.
“These are people who have gone through chemotherapy, lost their hair, and had terrible gastrointestinal side effects, toxicity in their nails, and all those other things,” Dr. Mittendorf said. “So a treatment that is basically not toxic is very attractive.”
Dr. Litton echoed Dr. Mittendorf’s sentiments. “It has not been a hard trial for people to become interested in. In fact, I’ve had several people come from different parts of the country just to be part of the trial,” Dr. Litton said. “And we really appreciate all the patients who have stepped forward to participate. It’s always important to encourage people to participate in clinical trials; otherwise we could never move forward with therapies such as this.”
These vaccines are not for everyone, however. Earlier clinical trials revealed that the peptide vaccines had limited efficacy in patients with late-stage, metastatic breast cancer.
“There’s a long list of reasons why these vaccines are not set up to be administered to patients who have diffusely metastatic disease,” Dr. Mittendorf said. “It would be difficult, with a peptide vaccine, to mount enough of an immune response to eradicate bulky disease. The microenvironment and immune environment around tumors change as tumors progress, so bulky metastatic tumors also have a less favorable environment for the immune system to function in. And a lot of patients with diffusely metastatic disease have received multiple lines of chemotherapy, which we suspect has a detrimental effect on the immune system.”
Future directions
The future of breast cancer vaccines holds many possibilities. Antigens such as cyclin E and folate-binding protein may be targeted for vaccination. Novel immunoadjuvants are being developed that may elicit an immune response more potent than that elicited by GM-CSF. And new approaches using vaccines and harnessing other aspects of the body’s immune system against recurrent breast cancer may be forthcoming.
“I would like to see some of these vaccines combined with other exciting immunotherapies that are coming on board,” Dr. Mittendorf said. For instance, a vaccine could be paired with a drug that inhibits CTLA-4, a protein that downregulates T cells. “Ipilimumab, an antibody that targets CTLA-4, could be used to take the brakes off the immune system. A vaccine would stimulate the T cells, and the anti–CTLA-4 treatment would allow them to proliferate,” she said.
Eventually, such vaccines could be used to treat patients much earlier in the course of their disease. “I think it would be an exciting route to look forward to in the frontline setting as well,” Dr. Litton said. “We could potentially cure more people up front at the time of diagnosis.”
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